It is known that conventional single-mode fibers (i.e. fibers not maintaining polarisation) used in optical-fiber communication systems have birefringence characteristics varying with both distance and time, thus causing the state of polarization of the signals propagating along the fiber to change in a continuous and unpredictable manner. In the case of coherent communication systems with heterodyne reception, which are the most widely used, the receivers can operate correctly only if the state of polarization of the received signal matches that of the signal emitted by the local oscillator; otherwise, only a part of the field undergoes heterodyne conversion. As a consequence even complete signal fading can occur. The importance of polarization-insensitive communication systems is therefore evident.
The proposed solutions to such a problem operate at either the receiving or the transmitting side.
The solutions based on intervention at the receiving side exploit polarization tracking receivers or polarization-diversity receivers. Polarization tracking receivers require an endless polarization transformer and a proper automatic control circuit; polarization-diversity receivers require two complete demodulating electronic stages in addition to a polarization splitter or, in case of balanced receivers, at least two such splitters and a polarization-independent 3 dB coupler. For actual introduction into industrial scale systems, the receivers should be monolithic integrated components, preferably of semiconductor material; however, at the present state of the art, both the development of integrated components carrying out the functions required with good performance, and the production of such units or an industrial scale with acceptable yield, present considerable difficulties. Furthermore, in one of the most interesting applications of coherent optical communication systems, such systems should be introduced into distribution networks, wherein a single source sends the information towards a plurality of receivers. In such a case the above-mentioned solutions have the further disadvantage that the devices rendering the system insensitive to polarization fluctuations ought to be associated with each receiver, thus increasing system complexity and costs.
The solutions based on interventions at the transmitting side are based on a fast switching of the polarization state of the signal to be transmitted. An example of these solutions is described by T. G. Hodgkinson, R. A. Harmona and D. W. Smith in the paper entitled Polarization-insensitive heterodyne detection using polarization scrambling. In this known system the transmitter comprises, between an amplitude modulator which modulates an optical carrier emitted by a laser with the data signal and the optical fiber, a polarization scrambler forcing the optical signal to switch between two orthogonal polarization states at a frequency equal to four times the symbol frequency. The polarization scrambler comprises a 1:1 fiber coupler sharing the signal in equal parts between two separate paths. One of these paths includes a waveguide phase modulator controlled by a square wave at the desired switching frequency, preceded by a polarization control device, which produces a correct state at the phase modulator input. The two paths join then at a second, polarization-selective coupler, to which the fiber is connected.
A device of this kind overcomes the disadvantages above, even if it entails the loss of half the power which can be received. In fact polarization switching is a simpler function than those carried out by endless polarization transformers, and can be implemented with devices which are less complex than those used in polarization diversity receivers and which are already available in integrated form. In addition, when used in a distribution network, the devices which render the system polarization-insensitive can be provided only in the transmitter, which is unique, and not in each receiver, so that the system complexity is not significantly increased.
Polarization switching gives rise to bandwidth problems at the receiver. In fact, the bandwidth required for the intermediate frequency filter in the receiver is approximately given by the bandwidth of the data signal (which, for an ASK system, as used in the system of the cited paper, has at least a main lobe whose width is equal to twice the symbol frequency) plus twice the switching frequency of the state of polarization. Spectrum broadening gives rise to many difficulties: 1) it is necessary to operate, at intermediate frequency, with electronic circuits with much wider bandwidth than is necessary in the absence of polarization switching; 2) the intermediate frequency, receiver is negatively affected by the very high level of the secondary lobes originating in the spectrum. Necessary to operate at rather high intermediate frequencies to prevent spectrum foldover around the origin of the frequencies from interfering with the main lobes of the useful signal; 3) a penalty is introduced in terms of the signal power necessary to obtain a certain error probability with respect to an ideal system operating without polarization switching. This penalty is due to the fact that the intermediate frequency filter requires greater signal amplitude and hence allows the passage of a larger amount of noise.
Operating at very high bit rates, like those generally envisaged for optical fiber systems (of the order of hundreds of Mbits/s or of Gbits/s) and at switching frequencies which are a multiple of the symbol frequency, as in the case of the above mentioned known system, the phenomena cited under 1) and 2) can make practical implementation of the system very difficult and expensive, or even impossible, since they involve the use of extremely fast electronics, up to the limits of the present technology. The phenomenon cited under 3) negatively affects system performance.